1. Field of the Invention
The present invention relates to an electroluminescence display device comprising electroluminescence elements and thin film transistors.
2. Description of the Prior Art
In recent years, electroluminescence (referred to herein after as “EL”) display devices comprising EL elements have gained attention as potential replacements for CRTs and LCDs. Research has been directed to the development of EL display devices using, for example, thin film transistors (referred to hereinafter as “TFT”) as switching elements to drive the EL elements.
FIG. 1 is a plan view showing a display pixel of an organic EL display device. FIG. 2A shows a cross-sectional view taken along line A—A of FIG. 1 while FIG. 2B shows a cross-sectional view taken along line B—B of FIG. 1.
As shown in these drawings, a display pixel 20 is formed in a region surrounded by a gate line GL and a data line DL. A first TFT serving as a switching element is disposed near an intersection of those lines. The source of the TFT 1 simultaneously functions as a second capacitor electrode 3 such that, together with a first capacitor electrode 2, it forms a capacitor 8. The source is connected to a gate electrode 15 of a second TFT 4 that drives the organic EL element. The source of the second TFT 4 contacts with an anode 6 of the organic EL element, while the drain of the TFT 4 is connected to a power source line (drive line) VL.
The first capacitor electrode 2, which is made of a material such as chromium, overlaps, over a gate insulating film 7, the second capacitor electrode 3 integral with the source of the first TFT 1. The first capacitor electrode 2 and the second capacitor electrode 3 together store charges with the gate insulating film 7 being interposed therebetween as a dielectric layer. The storage capacitor 8 serves to retain voltage applied to the gate electrodes 15 of the second TFT 4.
The first TFT 1, the switching TFT, will now be described.
First gate electrodes 11 made of refractory metal such as chromium (Cr) or molybdenum (Mo) are formed on a transparent insulator substrate 10 made of quartz glass, non-alkali glass, or a similar material. As shown in FIG. 1, the first gate electrodes 11 are integrally formed with the gate line GL such that a plurality of these electrodes extend from the gate line GL in the vertical direction in parallel with each other. Referring to FIG. 2A, the first capacitor electrode 2 formed in the same process as that of the first gate electrodes 11 is provided to the right side of the first gate electrodes 11. This first capacitor electrode 2, which constitutes the storage capacitor 8, has an enlarged portion between the first TFT 1 and the second TFT 4 as shown in FIG. 1 and is integral with a storage capacitor line CL extending therefrom in the directions.
A first active layer 12 composed of poly-silicon (referred to hereinafter as “p-Si”) film is formed on the gate insulating film 7. The first active layer 12 is of a so-called LDD (Lightly Doped Drain) structure. Specifically, low-concentration regions are formed on both sides of the gate. Source and drain regions, which are high-concentration regions, are further disposed on the outboard sides of the low-concentration regions. On the first active layer 12, a stopper insulating film.13 made of Si oxidation film is formed so as to prevent ions from entering the first active layer 12.
An interlayer insulating film 14 formed by sequential lamination of a SiO2 film, a SiN film, and a SiO2 film is provided on the entire surface over the gate insulating film 7, the active layer 12, and the stopper insulating film 13. The data line DL which functions as a drain electrode is electrically connected, through a contact hole C1 formed in the interlayer insulating film 14, to the drain in the active layer 12. A planarizing insulating film 18 made, for example, of an insulating organic resin is also formed over the entire surface for planarization.
In EL display devices which are driven by an electric current, the EL layers must have a uniform thickness. Otherwise, current concentration may occur in a portion of the layer having thinner thickness. Thus, a significantly high level of planarity is required at least in portions where the EL elements are to be formed, and therefore the above-described planarizing film 18 made of a material having fluidity prior to hardening is employed.
The second TFT 4 which drives the organic EL element will be described with reference to FIGS. 1 and 2B.
On the insulating substrate 10, second gate electrodes 15 made of the same material as the first gate electrodes 11 are provided, and a second active layer 16 is further formed on the gate insulating film 7. Then, a stopper insulating film 17 is formed on the second active layer 16 in a manner similar to the above-mentioned stopper insulating film 13.
Intrinsic or substantially intrinsic channels are formed in the second active layer 16 above the gate electrodes 15, and source and drain regions are formed on respective sides of these channels by doping p-type impurities, thereby constituting a p-type channel TFT.
The above-described interlayer insulating film 14 is provided on the entire surface over the gate insulating film 7 and the second active layer 16, and the power source line VL is electrically connected, through a contact hole C2 formed in the interlayer insulating film 14, to the drain in the active layer 16. Further, the planarizing film 18 is formed over the entire surface, such that the source is exposed through a contact hole C3 formed in the planarizing film 18 and the interlayer insulating film 14. A transparent electrode made of ITO (Indium Tin Oxide) that contacts the source through this contact hole C3, namely, the anode 6 of the organic EL element 20, is formed on the planarizing insulating film 18.
The organic EL element 20 is formed by laminating, in order, the anode 6, an emissive element layer EM comprising a first hole transport layer 21, a second hole transport layer 22, an emissive layer 23 and an electron transport layer 24, and a cathode 25 made of a magnesium-indium alloy. The cathode 25 is substantially disposed over the entire surface of the organic EL elements.
The principle and operation for light emission of the organic EL element is as follows. Holes injected from the anode 6 and electrons injected from the cathode 25 recombine in the emissive layer 23, to thereby excite organic molecules constituting the emissive layer 23, thereby generating excitons. Through the process in which these excitons undergo radiation until deactivation, light is emitted from the emissive layer. This light radiates outward through the transparent anode via the transparent insulator substrate and resultant light emission is observed.
The above-described EL elements are expected to be actively developed, and pixel size must be minimized to the greatest possible extent in order that the number of pixels disposed in a limited display pixel area can be maximized to achieve higher resolutions.
This requirement will be described with reference to FIG. 1. Various spaces must be reduced, such as the space between the anode 6 and the second gate electrode 15, the space between the anode 6 and the gate line GL for the pixel located in the next row, and the space between the storage capacitor 8 and the anode 6.
However, because the EL element is a self emissive element, a leak current is generated at the TFT when the light emitted by the element is introduced into the active layer of the TFT, whereby the amount of current supplied to the EL element is increased and the luminance is higher than actually required for that EL element. Consequently, the display emission can not accurately realize the display data.
In monochrome displays, this problem manifests itself as a situation in that display areas that should be gray are displayed a whiter shade than intended.